This invention relates to riveting processes and apparatus and, more particularly, to riveting processes and apparatus wherein a nonheaded rivet slug is squeezed between a pair of rams.
Several squeeze-type riveting processes using nonheaded rivet slugs are known in the prior art. In many of these processes, a gap must exist between the items being riveted together and the upper clamp that holds the workpiece during part of the rivet forming process, in order to provide room for the upper rivet head to be formed. Such prior art methods of squeeze-type riveting generally use either a machine having a floating upper clamp or a retractable upper clamp to attain the desired gap. However, many riveting machines exist that have a fixed upper clamp assembly, which, in the past, has prevented them from using nonheaded rivet slugs. As a result, such machines have been required to use expensive, headed rivet blanks. Obviously, it would be desirable to modify the operation of such machines in a manner that would allow them to be used with inexpensive nonheaded rivet slugs.
In this regard, prior art riveting machines that have fixed upper clamps and use nonheaded rivet slugs are known. These machines operate in accordance with what is known as the squeeze-vibrate riveting process. In the squeeze-vibrate process the lower head of the rivet is formed by exerting a squeeze force on the lower end of the rivet slug. The upper head is formed by a series of sharp, hammer-like blows on the upper end of the rivet slug by the upper ram. The primary disadvantage of rivet joints formed using the squeeze-vibrate process is that they have a fatigue life that is lower than rivet joints in which both rivet heads are formed entirely by squeeze forces, that is, by a squeeze-squeeze process. Another disadvantage is that the vibrate force is difficult to accurately regulate.
The lower fatigue life of rivet joints formed by the squeeze-vibrate process is due to the nonlaminar interference pattern which forms in the workpiece adjacent the rivet hole during the vibrate portion of the riveting process. As the rivet slug is deformed to form the rivet heads, the shank of the slug within the rivet hole in the workpiece expands radially. As the slug expands, it first fills the rivet hole and then expands the hole slightly, forming a rigid joint. In the vibrate portion of the squeeze-vibrate process, force is applied to the upper end of the rivet slug in short, high-energy strokes. The periodic application of energy causes the expansion of the slug to occur in small increments. The series of short, high-energy impulses causes a step differential (nonlaminar) expansion of the rivet at the work piece interface. During use of the workpiece after assembly, fatigue cracks originate at the interface and cause deterioration of the rivet joint. The rivet joint thus loses its strength in a relatively short time. Consequently, it is desirable to eliminate the vibrate portion of the riveting process and use a squeeze-squeeze process, since a squeeze-squeeze process produces rivet joints that are less likely to have nonlaminar interference patterns.
It is therefore an object of this invention to provide a new and improved high fatigue, slug squeeze riveting process and apparatus using nonheaded rivet slugs.
It is a further object of this invention to provide a process and apparatus which will allow high fatigue, slug squeeze riveting to be accomplished on machines having a fixed upper clamp using nonheaded rivet slugs.
It is another object of this invention to provide apparatus adapted to modify existing machines having fixed upper clamps so as to make such machines useful for producing high fatigue, slug squeeze rivet joints using nonheaded rivet slugs.